The present invention relates to a method for producing of a polybutylene terephthalate and more particularly to a method for producing of a polybutylene terephthalate (hereinafter referred to as "PBT") from terephthalic acid (hereinafter referred to as "TPA") and 1,4-butanediol (hereinafter referred to as "BG") both as essential monomers according to a direct polymerization process, in which BG is fed in steps to an esterification reaction process, i.e. a part of BG is added prior to the initiation of the reaction and the remaining is added during the reaction, which is preferably carried out under reduced pressure, thereby decreasing the amount of BG used in the esterification and at the same time decreasing the amount of tetrahydrofuran byproduced to obtain PBT of a good quality in a commercially advantageous manner.
PBT has heretofore been used widely as not only engineering plastics but also fibers, films and other functional resins because of its superior mechanical, chemical and physical properties.
There are two commercial or industrial manufacturing methods of PBT, one being a direct polymerization process which employs TPA as a starting dicarboxylic acid and a transesterification process which employs a lower alkyl ester of TPA. It is said that in the latter manufacturing process using such lower alkyl ester the polymer manufacturing cost is high in comparison with the direct polymerization process because the process using such lower alkyl ester goes through a transesterification process which is not found in the direct polymerization process and therefore the manufacturing process and conditions become complicated.
On the other hand, according to the direct polymerization process, BG as a reactant easily undergoes a cyclization reaction and is converted to tetrahydrofuran (hereinafter referred to as "THF") in an esterification reaction, so the esterification of TPA and BG will not proceed unless a large amount of BG is fed in this esterification reaction, thus resulting in that a large amount of BG is consumed and it is expensive to separate and recover the byproduced THF. This is a serious problem in industrial production.
From the standpoint of reducing the amount of BG used and suppressing the by-production of THF there have been proposed PBT manufacturing methods using one or both of an organotitanium compound and an organotin compound as a catalyst used in the direct polymerization process (see, for example, Japanese Pat. Publication No. 30010/80 or U.S. Pat. No. 3,936,421).
However, even if such a catalyst is selected, there is a limit in the reduction of the amount of BG used in the direct polymerization process, and where the total amount of BG used relative to TPA is small, the time required for the esterification reaction prolongs to a great extent and as the case may be the reaction is not completed, while even where the reaction is completed, a subsequent polycondensation reaction may not afford a polymer of a satisfactory quality.
On the other hand, the method of producing a polyethylene terephthalate (hereinafter referred to as "PET") using TPA and ethylene glycol (hereinafter referred to as "EG") as starting materials according to the above direct polymerization process is widely practised industrially as a PET manufacturing method although the polymer to be produced is different from that in the PBT manufacturing method of the present invention. In the direct polymerization process adopted in this PET manufacturing method, diethylene glycol (hereinafter referred to as "DEG") is byproduced DEG (see Japanese Patent Laid Open No. 41794/76).
However, between the direct polymerization process for PET and that for PBT there are differences in physical and chemical properties of the starting glycol components (EG and BG). For example, the boiling point of EG is 195.degree. C. and according to ordinary esterification reaction conditions there is adopted a temperature (e.g. 220.degree.-280.degree. C.) above the boiling point of EG, so it is desirable to adopt a pressurized condition, while the boiling point of BG is 235.degree. C. and an ordinary esterification reaction can be effected to a satisfactory extent event at a temperature (e.g. 180.degree.-240.degree. C.) below the boiling point of BG.
Besides, the production of PET and that of PBT according to the direct polymerization process are the same in that the esterification reaction is performed using a large excess of glycol relative to TPA. But in the direct polymerization process for PBT, it becomes very easy for BG to decompose in the presence of TPA, resulting in that BG is easily converted to THF, thus causing a lack of the starting material used for acceleration of the reaction. In the direct polymerization for PET, the decomposition of EG does not occur but DEG is produced and copolymerized with PET, thus resulting in deterioration of the quality of the polymer obtained. Thus, there are problems quite different from each other between the production of PET and that of PBT.
Further, in the direct polymerization process for PET there usually is adopted a means in which the divided addition of EG in its esterification reaction is made not to a reaction system of TPA alone but to a mixture of TPA and bis(hydroxyethyl)terephthalate (hereinafter referred to as "BHT") which is an initial reaction product of TPA and EG, as in the invention disclosed in the foregoing Japanese Patent Laid Open No. 41794/76).
As to the starting TPA, moreover, acetic acid used in an air oxidation of p-xylene remains in TPA which is produced by such air oxidation; more particularly, a small amount, usually about 0.003-0.3 wt. %, of such acetic acid is contained in the TPA. Where TPA containing such acetic acid is used as a starting material, the acetic acid easily reacts with BG to form glycol esters thereof such as mono- and diacetate of BG. These glycol esters of acetic acid have boiling points very close to the boiling point of BG and so it is impossible to easily separate the two in the recovery of BG. Particularly, in the case where the recovered BG is recycled and reutilized, glycol esters of acetic acid are gradually accumulated in BG and the concentration thereof becomes higher to the extent that it impedes the completion of the esterification reaction and the quality of the resulting PBT is impaired.